For example, I'm a married man. I am married because I want to share my life experiences with my wife. I want to cooperate with her in building those experiences and navigating through the options. I engage in this deep cooperation and sharing with another person precisely because I am selfish. I give money and sometimes my time to causes like the Red Cross and Institute for Justice, again, not because I'm naturally cooperative, but because I view, over the long term, that these organizations may actually either be helpful to me someday or will help mold the kind of society I want to live in: I'm being cooperative because this sort of cooperation is ultimately in my best interest... in other words, I'm being cooperative precisely because I am being selfish.
I could rob a bank to get cash now, cheat a merchant that gives me too much change, simply be rude to people because it feels good in any one moment, or... as the test in the article states... take from the communal pot without contributing anything. That's the sort of thing the article is calling selfish, but looking at my life as a whole, each one of those things actually aren't all that selfish; they simply satiate in the moment and comes with consequences if one things more clearly about it. Each of those "quick fixes" end up causing me more long term harm than good (always being on the run, making a society where dishonesty is the norm, encouraging everyone to be rude, encouraging less sensible generosity).
So yeah, we may cooperate as a default, we may make short-sighted self-interested decisions with a little thought, but selfish cooperation takes the most thinking.